A troopship (also troop ship or troop transport or trooper) is a ship used to carry soldiers, either in peacetime or wartime. Operationally, standard troopships – often drafted from commercial shipping fleets – cannot land troops directly on shore, typically loading and unloading at a seaport or onto smaller vessels, either tenders or barges.

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Ships to transport troops were already used in Antiquity. Ancient Rome used the navis lusoria, a small vessel powered by rowers and sail, to move soldiers on the Rhine and Danube.[1]

Nicknamed the "Grey Ghost", RMS Queen Mary holds the all-time record for most troops on one passage, 15,740 on a late July 1943 run from the U.S. to Europe[2]

The modern troopship has as long a history as passenger ships do, as most maritime nations enlisted their support in military operations (either by leasing the vessels or by impressing them into service) when their normal naval forces were deemed insufficient for the task. In the 19th century, navies frequently chartered civilian ocean liners, and from the start of the 20th century painted them gray and added a degree of armament; their speed, originally intended to minimize passage time for civilian user, proved valuable for outrunning submarines and enemy surface cruisers in war. HMT Olympic even rammed and sank a U-boat during one of its wartime crossings. Individual liners capable of exceptionally high speed transited without escorts; smaller or older liners with poorer performance were protected by operating in convoys.

Most major naval powers in the late 19th and early 20th centuries provided their domestic passenger lines with subsidies to ensure ships capable of transporting troops would be available during wartime. The British government aided both Cunard and the White Star Line to construct the liners RMS Mauretania, RMS Aquitania, RMS Olympic, RMS Britannic. When the vulnerability of these ships to return fire was realized most were used instead as troopship or hospital ships.

The RMS Queen Mary and the RMS Queen Elizabeth were two of the most famous converted liners of World War II. When they were fully converted, each could carry well over 10,000 troops per trip. Queen Mary holds the all-time record, with 15,740 troops on a single passage in late July of 1943,[2] transporting a staggering 765,429 military personnel during the war.[2]

The modified Liberties were capable of transporting up to 450,[4] 550,[5] or 650[6] (sources vary) troops or prisoners-of-war. Modifications included installation of bunks stacked five deep on the forward tweendeck, additional shower and head facilities, two additional diesel-powered generators,[5] and installation of two more Oerlikon 20-mm automatic cannons.[4][5][7][8]

The designation HMT (Hired Military Transport) would normally replace RMS (Royal Mail Ship) or SS (Steamship) on ships converted to troopship duty with Great Britain's Royal Navy. The US used two designations: WSA for troopships operated by the War Shipping Administration using Merchant Marine crews, and USS (United States Ship) for vessels accepted into and operated by the United States Navy. Initially troopships adapted as attack transports were designated AP; starting in 1942 keel-up attack transports received the designation APA.

^"HAER for Private Frederick C. Murphy". United States Maritime Administration. Retrieved 6 August 2013. "In the summer of 1945, eighty-four VC2-S-AP2 Victory ships, including the Maritime Victory, were converted into troopships by MARITIME VICTORY the U.S. Maritime Commission in preparation for an assault on the Japanese home islands. The ship made several crossings of the Atlantic Ocean and was used to repatriate American troops from Europe after World War II. pp. 1-2